A known type of robotic arm is able to follow a path to a work site with the body of the arm remaining on the path. This is known as “tip following”. That is, the body of the arm follows the path taken by the tip, in a snake like manner. This may include, for example, an apparatus comprising a support arm for a work head that is to be positioned at a location. This may further include operating means for operational control of the work head and control means for controlling the attitude and positioning of the arm wherein the support arm comprises at least one segment having a plurality of links, each link being articulated with respect to its neighbor and means for controlling the position and/or attitude of the segment relative to a datum thereby enabling the arm to follow and adapt to a predetermined path within the machine from entry in the machine housing to the work site. Each segment comprises a plurality of links, there being a degree of articulation between adjacent inks. By maintaining articulation of the links in each segment under tension, the spatial positioning of each segment may be controlled with precision to enable the arm to follow a convoluted path to guide the work tool into the machine. An arm of this type is as stated above, know as a “tip following” arm because of its ability to extend axially of itself in the manner of a snake and to flow around obstacles in order to follow a convoluted path. The tip may further carry a work took and for certain tasks, it is necessary for the arm to follow the path precisely, and for the tip to be steady and stable. If a process performed at the tip involves loads which are transmitted to the arm, the arm must also be appropriately stiff or the tip to remain steady. Examples of such tasks includes locating surgical tools at the tip of the arm for microscopic surgery through an incision or natural orifice of the body, or conducting repair work within a compressor of a jet engine
An arm for performing this type of task is described in WO 02/100608. Such an arm typically comprises a plurality of sequentially arranged articulated links, with control cables passing from actuators located at the proximal end of the arm, through successive links to terminate each at a more distal control link, for controlling that link. For example, a set of three cables may terminate at a particular link at positions around the circumference of the link. Differential movement of the actuators of the set of cables terminating at a particular link exerts a moment on the link which results in a change of the arm shape. The links may be grouped together, with a group of passive links terminating in a control link is known as a segment, and an arm may have several such controllable segments. Alternatively, each link may be directly controlled. Typically, the apertures in the more proximal links through which the control cables pass are located near the periphery of each link. The actuators may be controlled by a computer control system to adjust the arm shape.
In order to provide a structure that is stable and can be manipulated precisely, it has previously been the thought that the cables should fit closely within the apertures passing through the various links. This is so that the links are restricted from moving in relation to the cables, so their position is more accurately determinable. If the cables are a loose fit within the apertures then, particularly when the arm is straight or passes through the straight position, it is possible that the links will be able to move in an uncontrolled or unpredictable manner. The device will therefore provide lower stiffness, and lower position accuracy, and may further exhibit hysteresis errors. However, because of engineering constraints, such as manufacturing tolerances, the need to push the cables through the apertures when the arm is constructed, the tendency of cables to change diameter with changing load, and the need to be able to move the cables within the apertures when the arm is in use, in practice it has been determined that the apertures should be provided slightly larger than the cable diameter. This, however, inevitably results in a small error in the positioning of the arm, which is difficult to calculate and take account of in controlling the arm shape.